This invention relates generally to microelectronic structures and devices and to a method for their fabrication, and more specifically to acoustic wave devices and to the fabrication and use of acoustic wave devices, and to monolithic integrated circuits that include acoustic wave devices.
Acoustic wave devices have several applications in the microelectronics industry. For example, acoustic wave devices can be used to perform active or passive signal processing functions suitable for delay lines, attenuators, phase shifters, filters, amplifiers, oscillators, mixers, limiters, and the like. Such acoustic wave devices are often integrated with other microelectronic components such as integrated circuits and RF generators to form assemblies for telecommunication, digital processing, and other applications.
Acoustic wave devices, such as surface acoustic wave and bulk acoustic wave devices, include a transducer coupled to piezoelectric material that converts an electronic signal received from the transducer to an acoustic wave. The acoustic wave devices are often fabricated by forming the transducer on bulk piezoelectric material or on a thin-film of piezoelectric material formed over a substrate such as a sapphire. Attempts have also been made to grow thin-film piezoelectric material over a semiconductor substrate. Formation of such films on semiconductor substrates is desirable because it allows for integration of the acoustic wave devices with other microelectronic devices on a single substrate. However, thin films of piezoelectric material formed on the semiconductor substrate are often of lesser quality than the bulk material because lattice mismatches between the host crystal and the grown crystal cause the grown thin film of piezoelectric material to be of low crystalline quality.
Generally, the desirable characteristics of acoustic wave devices increase as the crystallinity of the piezoelectric film increases. For example, the electromechanical coupling coefficient and the piezoelectric coefficient of a monocrystalline piezoelectric material is typically higher than the coefficient of the same material in polycrystalline or amorphous form. Accordingly, methods for forming monocrystalline piezoelectric films are desirable.
If a large area thin film of high quality monocrystalline piezoelectric material was available at low cost, a variety of semiconductor devices could advantageously be fabricated using that film at a low cost compared to the cost of fabricating such devices on a bulk wafer of the piezoelectric material or in an epitaxial film of such material on a sapphire substrate. In addition, if a thin film of high quality monocrystalline piezoelectric material could be realized on a bulk wafer such as a silicon wafer, an integrated device structure could be achieved that took advantage of the best properties of both the silicon and the piezoelectric material.
Accordingly, a need exists for a microelectronic structure that provides a high quality monocrystalline piezoelectric film over another monocrystalline material such as a semiconductor wafer and for a process for making such a structure.